/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alipay.sofa.ark.loader.jar;

import com.alipay.sofa.ark.loader.data.RandomAccessData;

import java.io.IOException;

/**
 * A ZIP File "End of central directory record" (EOCD).
 *
 * @author Phillip Webb
 * @author Andy Wilkinson
 * @see <a href="http://en.wikipedia.org/wiki/Zip_%28file_format%29">Zip File Format</a>
 */
final public class CentralDirectoryEndRecord {

    private static final int MINIMUM_SIZE           = 22;

    private static final int MAXIMUM_COMMENT_LENGTH = 0xFFFF;

    private static final int MAXIMUM_SIZE           = MINIMUM_SIZE + MAXIMUM_COMMENT_LENGTH;

    private static final int SIGNATURE              = 0x06054b50;

    private static final int COMMENT_LENGTH_OFFSET  = 20;

    private static final int READ_BLOCK_SIZE        = 256;

    private byte[]           block;

    private int              offset;

    private int              size;

    /**
     * Create a new {@link CentralDirectoryEndRecord} instance from the specified
     * {@link RandomAccessData}, searching backwards from the end until a valid block is
     * located.
     * @param data the source data
     * @throws IOException in case of I/O errors
     */
    public CentralDirectoryEndRecord(RandomAccessData data) throws IOException {
        this.block = createBlockFromEndOfData(data, READ_BLOCK_SIZE);
        this.size = MINIMUM_SIZE;
        this.offset = this.block.length - this.size;
        while (!isValid()) {
            this.size++;
            if (this.size > this.block.length) {
                if (this.size >= MAXIMUM_SIZE || this.size > data.getSize()) {
                    throw new IOException("Unable to find ZIP central directory "
                                          + "records after reading " + this.size + " bytes");
                }
                this.block = createBlockFromEndOfData(data, this.size + READ_BLOCK_SIZE);
            }
            this.offset = this.block.length - this.size;
        }
    }

    private byte[] createBlockFromEndOfData(RandomAccessData data, int size) throws IOException {
        int length = (int) Math.min(data.getSize(), size);
        return Bytes.get(data.getSubsection(data.getSize() - length, length));
    }

    public boolean isValid() {
        if (this.block.length < MINIMUM_SIZE
            || Bytes.littleEndianValue(this.block, this.offset, 4) != SIGNATURE) {
            return false;
        }
        // Total size must be the structure size + comment
        long commentLength = Bytes.littleEndianValue(this.block, this.offset
                                                                 + COMMENT_LENGTH_OFFSET, 2);
        return this.size == MINIMUM_SIZE + commentLength;
    }

    /**
     * Returns the location in the data that the archive actually starts. For most files
     * the archive data will start at 0, however, it is possible to have prefixed bytes
     * (often used for startup scripts) at the beginning of the data.
     * @param data the source data
     * @return the offset within the data where the archive begins
     */
    public long getStartOfArchive(RandomAccessData data) {
        long length = Bytes.littleEndianValue(this.block, this.offset + 12, 4);
        long specifiedOffset = Bytes.littleEndianValue(this.block, this.offset + 16, 4);
        long actualOffset = data.getSize() - this.size - length;
        return actualOffset - specifiedOffset;
    }

    /**
     * Return the bytes of the "Central directory" based on the offset indicated in this
     * record.
     * @param data the source data
     * @return the central directory data
     */
    public RandomAccessData getCentralDirectory(RandomAccessData data) {
        long offset = Bytes.littleEndianValue(this.block, this.offset + 16, 4);
        long length = Bytes.littleEndianValue(this.block, this.offset + 12, 4);
        return data.getSubsection(offset, length);
    }

    /**
     * Return the number of ZIP entries in the file.
     * @return the number of records in the zip
     */
    public int getNumberOfRecords() {
        long numberOfRecords = Bytes.littleEndianValue(this.block, this.offset + 10, 2);
        if (numberOfRecords == 0xFFFF) {
            throw new IllegalStateException("Zip64 archives are not supported");
        }
        return (int) numberOfRecords;
    }

}
